1995 454 Magnum EFI Stumbling and rough cold start
 

Hi all!

how do I go about troubleshooting my starboard engine? 5xx hours factory sealed Merc 454 Mag Efi bone stock. The port engine runs like a top!

It starts up rough and I need to throttle up the idle when it’s cold. Once it’s warm it idles fine but if I rev the engine in neutral quickly it will stumble and almost sound like it wants to backfire.

I assume I don’t notice it in gear because I would never punch the throttle that quickly so I don’t blow out the drives.

Here’s the work I’ve done so far:

new spark plugs gapped to .035” (old plugs looked fine just really old and slightly oily)
new engine oil and filter
new fuel filter
new riser gaskets (as preventative maintenance)
Compression test (cold) results shown below

I should have done a leak down test but I was so fed up with squeezing between the two engines,I called it done after the compression test.

https://cimg7.ibsrv.net/gimg/www.off...685d01d08.jpeg
All the plugs looked like this on both engines.


Cylinder 1 150

3 150

5 140

7 120

2 130

4 120

6 100

8 145
I’m hoping to at least get a season or two out of the boat before I have to refresh an engine.

I would start with a distributor cap, rotor and plug wires. Next would be pull the injectors and have them checked.

I would start with injectors. In my 30 years with my existing Merc any running issues have always been fuel related and not plug/wire/rotor etc.

Thank you for the suggestions.

its interesting that if I ease into the throttle the engine will rev up just fine. Its only an issue when you want to snap the throttle in neutral.

cap and rotor is cheap enough for try out.

I’ll have to do some research about the injectors! Gotta find a place that can clean them near me (so cal)

Where in So Cal....LA area or San Diego? RC Injection in Torrance does injector cleaning and flow testing. There are also places in San Diego

Those old plugs look horrible.
At least remove and inspect the dist caps.

I’m closer to LA!

Torrence is close enough. Thanks!

Could also be fuel pump...

Was thinking fuel issue myself. The reason is that you can ease it up and it won't stumble is the fuel demand is less than quick pull up to RPM

Pull the IAC and power cycle the key to the run position and see if its moving, Id hook a scanner to it and see whats going on

I’ve got the same exact engine.
bought this boat this winter because the guy I bought it from couldn’t get it running.
After all this cold/bad weather i finally got around to playing with it.
I checked compression.
mines a bit different than the above. Cylinder 1 190

Cylinder 3 170

Cylinder 5 175

Cylinder 7 165



Cylinder 2 175

Cylinder 4 185

Cylinder 6 180

Cylinder 8 190

next checked spark…got spark
all plugs looked decent and wires are connected to the correct holes.
next tried to fire her and she will fire on starting fluid but falls off
checked fuel pressure at the shrader valve…zero
next pulled fuel pump
it’s blown the hammer/foot that rides on the cam is worn down to a flat nothingness and the little bearing piece that rides on the cam is split in 2 down the middle..the pump wasn’t pumping at all
got a rebuild kit for the seawater pump and brand new fuel pump coming my way and hopefully I’ll get this big engine running the next few weeks
im betting yours is fuel pump related or check that little bearing on that cam
I would check fuel pressure at the shrader valve too

The same symptoms actually running? Or is this just on the trailer idling amd reving with no load?

Stumbles from throttle blips are often caused by ultra lean A/F charge. Where the accelerator pump or EFI injector charge of fuel is not sufficient.

If your EFI fuel pressure is low...the injectors will flow enough for a progressive thrittle increase, but unable to discharge on a throttle blip.

On that note, it's way harder to ignite a lean mixture...so weak spark is also a factor.

Lean mixtures have a longer burn time, so you get less cylinder pressure and less power.

Check your ignition timing to ensure it's within spec at idle and rev'd rpm. If the timing doesn't advance, a lot of the cyl pressure just goes out the exh valve.

yes so far this has been running on the trailer with muffs on the drives, reving with no load.

I took the boat on a very brief sea trial a month ago (sun was setting and I didn’t want to be the last one at the ramp) and in gear it seemed to accelerate well and plane out although I may have noticed that the engine in question required more throttle to match the Rpm’s of the other engine.

I’ve got the cap,rotor and new plug wires on the way. I’m going to swap them soon.

how can I verify the efi fuel pressure? I’m not near the boat currently but I’ll go look under the hatch for an access point later.

Most EFI fuel supply lines on engine will have a schreader valve test point. Find the fuel line routing and the stainless fuel tube. It will be on that. You need to have a fuel pressure tester gauge, that screws on and depresses the schreader valve.

The fuel pressure regulator may also be an issue.

Low fuel pressure issues can be from clogged fuel filters or water in the filter etc.